1. Technical Field
The present invention relates to mixed-signal circuits in general, and in particular to digital-to-analog converters. Still more particularly, the present invention relates to a current-steering type digital-to-analog converter.
2. Description of Related Art
Digital-to-analog converters (DACs) are electronic circuits for converting signals from a digital domain, where signals are typically represented by discrete quantities, to an analog domain, where signals are typically represented by continuous quantities. Well-known DAC types include resistor-string, charge-redistribution, and current-steering.
Any of the above-mentioned DAC types may use a binary-weighted design, a thermometer-code design, or a combination of both designs to convert a digital signal to an analog signal. With a binary-weighted design, analog signals are generated from an array of binary-weighted elements that can be capacitors, resistors, or current sources. The advantage of a binary-weighted design is compactness. With a thermometer-code design, analog signals are generated from an array of unit elements that can be capacitors, resistors, or current sources. Advantages of a thermometer-code design include monotonic input-output transfer characteristic and reduced glitch energy during switching transients.
For current-steering type DACs, it is often a sensible choice to have the most-significant bits (MSBs) implemented with the thermometer-code design, while the least-significant bits implemented with the binary-weighted design. The combination of the two designs is a tradeoff between implementation cost (such as chip size) and performance (such as improved monotonicity and reduced glitch energy).
One problem with conventional current-steering type DACs is that their internal bias points are often forced to change to accommodate output values during switching transients from one value to the next. Each time an internal bias point, such as the common-source node of the current-steering transistors, changes due to a change in the DAC's output value, parasitic capacitors associated with the internal bias node need If such charging or discharging affects the output signal in a non-linear way, the dynamic performance of the DAC will be compromised. Consequently, it is desirable to provide an improved current-steering type DAC.